^ Bit of a sidebar, but can you clarify what you mean about "Encounter at Farpoint" insisting the saucer has warp engines? I didn't recall it going to warp at all on its own. It catches up to the stardrive section at the planet, but I don't think it was made clear how far they were from the system when they had their encounter, so I wasn't left with any impression they had to have gone faster than light to get there in that time frame.
Sorry about the diversion. The thing is, the episode does show and tell us that our heroes in their combined starship fly 180 degrees away from their destination (and from Q who blocked the path) at maximum warp, faster than the ship has ever gone before, for several minutes. They then separate the saucer, spend some time playing with Q (time that Q could
arguably negate, reverse, tie into a pretzel or whatever), and then finish the journey to their destination in the stardrive section.
The saucer completes the same journey - which thus necessarily spans at least several extreme-warp-minutes, and must contradict the TNG TM claim that the saucer can only sustain warp for a couple of minutes. Apparently, the truth is that the saucer can sustain useful warp for a couple of hours
instead, at the very least.
It gets worse in "Arsenal of Freedom", where the saucer is sent on its merry way towards a distant starbase without
giving it a warp boost at combined mode. If the saucer indeed is capable of sustaining warp for several hours and spanning interstellar distances that way, then it would be criminally negligient of LaForge not to give it this boost. If, however, the saucer can attain warp all on its own, without boosts, the episode makes sense and LaForge is off the hook.
Since we know that the Feds can make just about anything go to warp, including near-derelicts and mining rigs, it is only logical that the saucer could
have warp, too. According to the combination of "EaF" and "AoF", it does.
(Ignore "EaF", though, and suddenly "AoF" no longer serves as proof. But ignoring "EaF" sounds a bit harsh, especially when the adventure was reaffirmed in the final episode of the show.)